1. Field of the Invention
This invention relates to metal-ceramic composite bodies produced by a reactive infiltration process, e.g., reaction-bonded bodies. More particularly, the invention relates to reaction-bonded silicon carbide having a boron carbide filler or reinforcement, and to ballistic armor structures produced from reaction-bonded silicon carbide bodies. The instant composite bodies are also extremely rigid, which in combination with their low specific gravity potential makes them attractive candidate materials for applications in precision equipment such as machines used to fabricate semiconductors.
2. Discussion of Related Art
In many applications, weight is not a critical factor, and traditional materials such as steel can offer some level of protection from airborne threats such as ballistic projectiles and shell fragments. Steel armors offer the advantage of low cost and the fact that they also can serve as structural members of the equipment into which they are incorporated. In recent decades, certain hard ceramic materials have been developed for certain armor applications. These ceramic-based armors, such as alumina, boron carbide and silicon carbide provide the advantage of being lighter in mass than steel for the same ballistic stopping power. Thus, in applications in which having an armor having the lowest possible mass is important, such as (human) body armor and aircraft armor, low specific gravity armor materials are called for. The lower the density, the greater the thickness of armor that can be provided for the same areal density. In general, a thick armor material is more desirable than a thinner one because a greater volume of the armor material can be engaged in attempting to defeat the incoming projectile. Moreover, the impact of the projectile on a thicker armor plate results in less tensile stress on the face of the plate opposite that of the impact than that which would develop on the back face of a thinner armor plate. Thus, where brittle materials like ceramics are concerned, it is important to try to prevent brittle fracture due to excessive tensile stresses on the back face of the armor body; otherwise, the armor is too easily defeated. Rather, by preventing such tensile fracture, the kinetic energy of the projectile perhaps can be absorbed completely within the armor body, which energy absorption manifests itself as the creation of a very large new surface area of the armor material in the form of a multitude of fractures, e.g., shattering.
U.S. Pat. No. 5,372,978 to Ezis discloses a projectile-resistant armor consisting predominantly of silicon carbide and made by a hot pressing technique. Up to about 3 percent by weight of aluminum nitride may be added as a densification aid. The finished product features a microstructure having an optimal grain size of less than about 7 microns. Fracture is intergranular, indicating energy-absorbing crack deflection. Moreover, the economics of manufacturing are enhanced because less expensive, less pure grades of silicon carbide can be used without compromising the structural integrity of the material.
U.S. Pat. No. 4,604,249 to Lihleich et al. discloses a composition particularly suited for armoring vehicles. The composition is a composite of silicon carbide and steel or steel alloy. Silicon and carbon particulates, optionally including silicon carbide particulates, are mixed with an organic binder and then molded to form a green body. The green body is then coked at a maximum temperature in the range of about 800° C. to about 1000° C. The temperature is then rapidly raised to the range of about 1400° C. to about 1600° C. under an inert atmosphere of at least one bar pressure. In this temperature range, the silicon and carbon react to form silicon carbide, thereby producing a porous body. The pores are then evacuated in a vacuum chamber, and the body is immersed in molten steel or steel alloy. The metal fills up the pores to produce a dense composite armor material.
U.S. Pat. No. 4,104,062 to Weaver discloses a high density, aluminum-modified boron carbide composition that is well suited as protective armor against ballistic projectiles. About 70 to 97 percent by weight of boron carbide powder is blended with about 3 to about 30 percent of aluminum powder. A temporary binder is added to this mixture, and a preform is pressed. This preform is then hot pressed in an oxygen-free atmosphere at a pressure of at least 500 psi (3.5 MPa) at a temperature of from 1800° C. to about 2300° C.
U.S. Pat. No. 3,857,744 to Moss discloses a method for manufacturing composite articles comprising boron carbide. Specifically, a compact comprising a uniform mixture of boron carbide particulate and a temporary binder is cold pressed. Moss states that the size of the boron carbide particulate is not critical; that any size ranging from 600 grit to 120 grit may be used. The compact is heated to a temperature in the range of about 1450° C. to about 1550° C., where it is infiltrated by molten silicon. The binder is removed in the early stages of the heating operation. The silicon impregnated boron carbide body may then be bonded to an organic resin backing material to produce an armor plate.
U.S. Pat. No. 3,859,399 to Bailey discloses infiltrating a compact comprising titanium diboride and boron carbide with molten silicon at a temperature of about 1475° C. The compact further comprises a temporary binder that, optionally, is carbonizable. Although the titanium diboride remains substantially unaffected, the molten silicon reacts with at least some of the boron carbide to produce some silicon carbide in situ. The flexural strength of the resulting composite body was relatively modest at about 140 MPa. A variety of applications is disclosed, including personnel, vehicular and aircraft armor.
U.S. Pat. No. 3,796,564 to Taylor et al., filed in 1967, discloses a hard, dense carbide composite ceramic material particularly intended as ceramic armor. Granular boron carbide is mixed with a binder, shaped as a preform, and rigidized. Then the preform is thermally processed in an inert atmosphere with a controlled amount of molten silicon in a temperature range of about 1500° C. to about 2200° C., whereupon the molten silicon infiltrates the preform and reacts with some of the boron carbide. The formed body comprises boron carbide, silicon carbide and silicon. Taylor et al. state that such composite bodies may be quite suitable as armor for protection against low caliber, low velocity projectiles, even if they lack the optimum properties required for protection against high caliber, high velocity projectiles. Although they desire a certain amount of reaction of the boron carbide phase, they also recognize that excessive reaction often causes cracking of the body, and they accordingly recognize that excessive processing temperatures and excessively fine-grained boron carbide is harmful in this regard. At the same time, they also realize that excessively large-sized grains reduce strength and degrade ballistic performance.
Each of the above-described armor inventions suffers from one shortcoming or another. Hot pressing is expensive and shape-limited. Hot pressed or sintered ceramics do not hold dimensional tolerances as well as reaction-bonded silicon carbide (“RBSC”). Iron matrix composite materials are heavy in relation to ceramic armors. The prior RBSC armors having a boron carbide reinforcement, sometimes referred to in this document as “reaction-bonded boron carbide” or “RBBC”, lose some of the boron carbide, particularly the finer particle sizes, due to reaction with the silicon infiltrant, as well as yield significant coarsening of the microstructure, as will be shown in more detail below. An infiltration temperature of 2200° C. is too high, and will likely result in exaggerated grain growth, also deleteriously coarsening the microstructure.
As the preceding synopsis of the patent literature indicates, reaction-bonded or reaction-formed silicon carbide has been proposed and evaluated as a candidate armor material as long ago as the 1960's.
In the Third TACOM Armor Coordinating Conference in 1987, Viechnicki et al. reported on the ballistic testing of a RBSC material versus sintered and hot pressed silicon carbide materials. Not only was the RBSC substantially inferior to the other silicon carbides, Viechnicki et al. came to the general conclusion that purer, monolithic ceramics with minimal amounts of second phases and porosity have better ballistic performance than multiphase and composite ceramics. (D. J. Viechnicki, W. Blumenthal, M. Slavin, C. Tracy, and H. Skeele, “Armor Ceramics—1987,” Proc. Third TACOM Armor Coordinating Conference, Monterey, Calif. (U.S. Tank-Automotive Command, Warren, Mich., 1987) pp. 27-53).
Accordingly, in spite of the price advantage of RBSC relative to sintered or hot pressed silicon carbide, what the market has preferred has been a sintered or hot pressed monolithic ceramic product. Until the introduction of the present assignee's product, there had been little or no RBSC armor on the market.
The details of a ballistic impact event are complex. One widely held theory of defeating a ballistic projectile is that the armor should be capable of fracturing the projectile, and then erode it before it penetrates the armor. Thus, compressive strength and hardness of a candidate armor material should be important. The above-mentioned armor patent to Taylor et al., for example, suggests a correlation between strength and ballistic performance. They noted that when the size of the largest grains exceeded 300 microns, both modulus of rupture and ballistic performance deteriorated. Keeping the size of the boron carbide grains below about 300 microns in diameter permitted their reaction-bonded boron carbide bodies to attain moduli of rupture as high as 260 MPa, and they recommended that for armor applications the strength should be at least 200 MPa.
There seems to be a consensus in the armor development community that hardness is indeed important in a candidate armor material, and in particular, that the hardness of the armor should be at least as great as the hardness of the projectile. As for the strength parameter, however, those testing armor materials have had a difficult time correlating mechanical strength (both tensile and compressive) with ballistic performance. In fact, except for hardness, there seems to be no single static property that functions as a good predictor of good armor characteristics in ceramic materials. Instead, the guidance that has been provided from the armor developers to the materials developers based upon actual ballistic tests has been that candidate armors in general should possess a combination of high hardness, high elastic modulus, low Poisson's ratio and low porosity. (Viechnicki et al., p. 32-33)
The instant inventors have re-visited RBSC, and even more particularly, RBBC as a candidate armor material because they believe that such a material can be developed whose anti-ballistic performance is competitive with other armor ceramics, such as the hot pressed armors, but at reduced cost.